1. Field of the Invention:
This invention relates to a plasma forming electrode used in an apparatus for high frequency plasma treatment, such as the formation of a film by plasma enhanced chemical vapor deposition (CVD) on the surface of a material, e.g. a semiconductor device, or for the subsequent plasma or reactive ion etching of the material surface.
2. Description of the Prior Art:
High frequency plasma treatment is a dry surface treatment which is carried out at a reduced pressure for various applications including the formation of a film on the surface of a material by plasma enhanced CVD, and the plasma etching or reactive ion etching (RIE) of the material surface. The treatment is carried out in an apparatus having a reduced pressure treating chamber provided with plasma forming electrodes. The material to be treated is positioned between the electrodes, and reactant or etching gases are supplied to form therebetween a high frequency plasma for performing the intended treatment.
The electrodes usually consist of an assembly of parallel flat electrodes. There are two types of assemblies used for different purposes. One of them is an assembly of the single substrate type and is used for processing a single substrate at a time. It comprises a pair of vertically spaced apart electrodes between which a high frequency plasma is produced to treat the material supported on the lower electrode. The other is a batch type of assembly which is used for the simultaneous processing of a plurality of substrates. It comprises a plurality of pairs of electrodes depending on the number of the substrates to be treated. This invention pertains to both of these two types of electrode assemblies.
The electrodes are made of aluminum, as described in, for example, Japanese Patent Application laid open under No. 56415/1986. There is also known an aluminum electrode having a surface layer formed by anodic oxidation in a bath containing sulfuric acid to ensure the production of a stable plasma, as described in, for example, "Solid State Technology, Japanese Edition", March, 1983, page 64, or Japanese Patent Application laid open under No. 58920/1988. Each electrode is in the shape of a disk having a diameter of, say, 130 to 200 mm and a thickness of, say, 2 to 10 mm.
Such apparatus are used for forming by plasma enhanced CVD an insulating film of silicon dioxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4), etc. on a substrate made of a single crystal of silicon (silicon wafer), or an amorphous silicon film on a substrate. A silicon dioxide (SiO.sub.2) film is usually deposited from a mixture of TEOS (tetraethyl orthosilicate) and oxygen (O.sub.2) gases, and a silicon nitride (Si.sub.3 N.sub.4) film from a mixture of monosilane (SiH.sub.4) gas with ammonia (NH.sub.3) or nitrogen (N.sub.2) gas. The gases are supplied into a treating chamber having a vacuum present of, say, 15 to 20 torr and an electric current having a high frequency of, for example, 13.56 MHz is applied to produce a plasma between the electrodes. The gases react and deposit a reaction product forming an insulating film on a silicon wafer heated in the range of 300.degree. to 400.degree. C.
The reaction product, such as silicon dioxide or nitride, is, however, deposited also on the electrodes, the wall surfaces of the treating chamber, etc., although only in a small amount. Even so, the deposition of the reaction product on the electrodes, especially the upper electrode, among others, exerts a serious effect on the efficiency of treatment, as it prevents the formation of a stable plasma and thereby lowers the yield of films having good quality.
The treating chamber is, therefore, cleaned before each cycle of treatment is started. There is known a cleaning method which employs a nitric acid bath to remove the deposit by etching. This method, however, also seriously etches the aluminum electrodes and causes a heavy consumption thereof. Accordingly, there is also known a dry cleaning method which employs a fluorine containing gas, such as CF.sub.4, C.sub.2 F.sub.6, C.sub.3 F.sub.8 or NF.sub.3 gas, for etching away the deposit. This method is widely adopted, it can keep the electrode consumption at a relatively low level and moreover enables the cleaning of the apparatus in situ. The dry cleaning of a treating chamber is carried out by supplying it with etching gas and igniting a high frequency plasma discharge. The reaction product which has been deposited on various portions of the chamber is converted to low boiling point fluoride compounds by the chemical reaction of the resulting radicals and the sputtering effect of ions, and the fluoride is removed by vaporization from their surfaces.
The plasma or reactive ion etching treatment is employed for a variety of purposes in a semiconductor manufacturing process including the formation of a circuit and a dopling layer. For example, a high frequency plasma discharge in the presence of CF.sub.4, CHF.sub.3, C.sub.2 F.sub.6, NF.sub.3, SiF.sub.4, SF.sub.6 or other fluorine containing gas under a low pressure in the order of about 0.01 to several torr is employed for the radical or reactive ion etching into a predetermined shape of not only the silicon of a silicon wafer, but also an ininsulating film of silicon dioxide or nitride formed thereon. The high frequency plasma treatment in the presence of a fluorine containing gas is repeated for any such purpose as has hereinabove been stated.
However, aluminum electrodes having a surface layer formed by anodic oxidation in a bath containing sulfuric acid which are widely used in apparatus for high frequency plasma treatment suffer a heavy reduction in layer thickness due to corrosion during the treatment. Therefore, the development of an aluminum electrode having an anodized surface layer of high durability has been eagerly awaited.